The present invention relates to a metallic gasket for use in an internal combustion engine, and more particularly to a metallic gasket to be interposed between a cylinder block and a cylinder head.
Between a cylinder block and a cylinder head of an internal combustion engine is interposed a gasket for the purpose of sealing a gap between joining surfaces of these members to maintain airtightness of a cylinder and also to prevent leakage of coolant water and engine oil. As shown in FIG. 1 (which corresponds to FIG. 2 of Laid-Open Japanese Patent Specification No. 59-97363 (1984)), in gasket 01 is formed an upwardly swelled portion (this is called "embossment" throughout this specification and claims) along an edge 03 of a gasket opening 02 corresponding to a cylinder bore of a cylinder block in order to deal with machining errors of joining surfaces of a cylinder block and a cylinder head and/or discrepancy in thermal expansion caused by uneven temperature distribution in the cylinder block and the cylinder head during operation of the engine.
The embossment 04 is subjected to compression load due to fastening forces of steel bolts for connection of the cylinder block and the cylinder head, and since this compression load would vary due to compression and explosion of intake gas and expansion of combustion gas which are quickly repeated within a cylinder bore, a large repetitive stress is generated in the embossment 04. Especially in the case where aluminium, aluminium alloy or like material is employed for a cylinder block and a cylinder head in order to realize a light weight engine, in the event that the internal combustion engine assumes a high power rotating condition or a high speed rotating condition and thus temperature rise of the engine body becomes substantial, a stress level of the above-mentioned repetitive stress would become high because a coefficient of thermal expansion of the above-mentioned material is larger than that of the steel bolts. Therefore, if the embossment 04 is made higher, a larger fastening force of the bolt is needed, causing the material of the gasket to be degraded compared with the customary gasket, and fatigue rupture of the embossment 04 is liable to occur.
A plan configuration of the embossment 04 takes a shape of a closed loop surrounding the gasket opening 02, for instance, a circular shape, and with respect to a cross-section configuration, in the prior art a rising angle .alpha. in an inner circumferential portion was equal to a rising angle .beta. in an outer eircumferential portion, and a crest portion of the embossment was gently curved. In the event that an embossment 04 having the above-mentioned construction is subjected to compression load in the vertical directions as described above, a stress generated in the inner circumferential portion becomes larger than a stress generated in the outer circumferential portion, accordingly the shape of the inner circumferential portion is deformed, and a sealing property is degraded.